1. Technical Field
The present disclosure relates to advantageous methods and apparatus for spinal stabilization. More particularly, the present disclosure relates to methods and apparatus for providing dynamic stabilization to the spine so as to provide clinically efficacious results.
2. Background Art
Low back pain is one of the most expensive diseases afflicting industrialized societies. With the exception of the common cold, it accounts for more doctor visits than any other ailment. The spectrum of low back pain is wide, ranging from periods of intense disabling pain which resolve to varying degrees of chronic pain. The conservative treatments available for lower back pain include: cold packs, physical therapy, narcotics, steroids and chiropractic maneuvers. Once a patient has exhausted all conservative therapy, the surgical options generally range from micro discectomy, a relatively minor procedure to relieve pressure on the nerve root and spinal cord, to fusion, which takes away spinal motion at the level of pain.
Each year, over 200,000 patients undergo lumbar fusion surgery in the United States. While fusion is effective about seventy percent of the time, there are consequences even to these successful procedures, including a reduced range of motion and an increased load transfer to adjacent levels of the spine, which may accelerate degeneration at those levels. Further, a significant number of back-pain patients, estimated to exceed seven million in the U.S., simply endure chronic low-back pain, rather than risk procedures that may not be appropriate or effective in alleviating their symptoms.
New treatment modalities, collectively called motion preservation devices, are currently being developed to address these limitations. Some promising therapies are in the form of nucleus, disc or facet replacements. Other motion preservation devices provide dynamic internal stabilization of the injured and/or degenerated spine, e.g., the Dynesys stabilization system (Zimmer, Inc.; Warsaw, Ind.) and the Graf Ligament. A major goal of this concept is the stabilization of the spine to prevent pain while preserving near normal spinal function. The primary difference in the two types of motion preservation devices is that replacement devices are utilized with the goal of replacing degenerated anatomical structures which facilitate motion while dynamic internal stabilization devices are utilized with the goal of stabilizing and controlling abnormal spinal motion.
Over ten years ago a hypothesis of low back pain was presented in which the spinal system was conceptualized as consisting of the spinal column (vertebrae, discs and ligaments), the muscles surrounding the spinal column, and a neuromuscular control unit which helps stabilize the spine during various activities of daily living. Panjabi M M. “The stabilizing system of the spine. Part I. Function, dysfunction, adaptation, and enhancement.” J Spinal Disord 5 (4): 383-389, 1992a. A corollary of this hypothesis was that strong spinal muscles are needed when a spine is injured or degenerated. This was especially true while standing in neutral posture. Panjabi M M. “The stabilizing system of the spine. Part II. Neutral zone and instability hypothesis.” J Spinal Disord 5 (4): 390-397, 1992b. In other words, a low-back patient needs to have sufficient well-coordinated muscle forces, strengthening and training the muscles where necessary, so they provide maximum protection while standing in neutral posture.
Dynamic stabilization (non-fusion) devices need certain functionality in order to assist the compromised (injured or degenerated with diminished mechanical integrity) spine of a back patient. Specifically, the devices must provide mechanical assistance to the compromised spine, especially in the neutral zone where it is needed most. The “neutral zone” refers to a region of low spinal stiffness or the toe-region of the Moment-Rotation curve of the spinal segment (see FIG. 1). Panjabi M M, Gael V K, Takata K. 1981 Volvo Award in Biomechanics. “Physiological Strains in Lumbar Spinal Ligaments, an in vitro Biomechanical Study.” Spine 7 (3): 192-203, 1982. The neutral zone is commonly defined as the central part of the range of motion around the neutral posture where the soft tissues of the spine and the facet joints provide least resistance to spinal motion.
This concept may be visualized with reference to load-displacement or moment-rotation curves for an intact spine and an injured spine, as shown in FIG. 1. The curves are non-linear; that is, the spine mechanical properties change with the amount of angulations and/or rotation. If the curves on the positive and negative sides are understood to represent spinal behavior in flexion and extension, respectively, then the slope of the curve at each point represents spinal stiffness. As seen in FIG. 1, the neutral zone is the low stiffness region of the range of motion.
Experiments have shown that after an injury to the spinal column and/or degeneration of the spine, neutral zones, as well as ranges of motion, increase (see FIG. 1). However, the neutral zone increases to a greater extent than does the range of motion, when described as a percentage of the corresponding intact values. This implies that the neutral zone may be a better measure of spinal injury and instability than the range of motion. Clinical studies have also found that range of motion does not correlate well with low back pain. Therefore, an unstable spine needs to be stabilized, especially in the neutral zone.
With the foregoing in mind, those skilled in the art will understand that a need exists for spinal stabilization devices, systems and/or methods that overcome the shortcomings of prior art devices, systems and methods. The present invention provides devices, systems and methods for enhanced and efficacious spinal stabilization. More particularly, the present disclosure provides advantageous dynamic internal stabilization devices, systems and methods that are flexible so as to move with the spine, thus allowing the disc, the facet joints, and the ligaments normal (or improved) physiological motion and loads necessary for maintaining their nutritional well-being. The devices, systems and methods of the present disclosure also advantageously accommodate different physical characteristics of individual patients and anatomies to achieve a desired and/or improved posture for each individual patient.